Ethylene is an important petrochemical. In 1998 about 80 million tons of ethylene was produced, and demand is expected to reach 100 million tons by 2003. The primary use for ethylene is as a monomer for the production of low and high density polyethylene. Approximately 60% of world ethylene consumption goes into making polyethylene for such products as plastic films, containers, and coatings. Other uses for ethylene include the production of vinyl chloride, ethylene oxide, ethylbenzene and alcohols. Presently, about 90% of the ethylene is produced by the steam cracking of light paraffin, naptha, and gas oil.
Propylene is another important raw material. In 1998 about 46 million tons of propylene was produced, and demand is expected to reach 60 million tons by 2003. About 55% of the world consumption is directed to the production of polypropylene. Other important end products using propylene include acrylonitrile for acrylic and nylon fibers, and propylene oxide for polyurethane foams. About two-thirds of the propylene is produced from steam cracking, and the remaining third as a by-product of FCC gasoline refining.
A potential alternative to producing ethylene and propylene from petroleum feedstock is to use an oxygenate feedstock. Particularly promising oxygenate feedstocks are alcohols, such as methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates can be produced from a variety of sources including synthesis gas derived from natural gas; petroleum liquids; and carbonaceous materials, including coal. Because of the wide variety of these relatively inexpensive sources, alcohol and other oxygenates have promise as an economical, non-petroleum source for ethylene and propylene production.
One way of producing ethylene and propylene is by the catalytic conversion of methanol using a silicoaluminophosphate (SAPO) molecular sieve catalyst. For example, U.S. Pat. No. 4,499,327 discloses making olefins from methanol using SAPO molecular sieve catalysts. The advantage of using SAPO catalysts is that such catalysts have relatively high ethylene and propylene selectivities.
U.S. Pat. No. 6,049,017 teaches the use of a SAPO-34 catalyst to first convert methanol to an olefin hydrocarbon product. The ethylene and propylene are then separated from the olefin hydrocarbon product. Di-olefin, such as butadiene, is selectively hydrogenated, and any isobutene is catalytically converted to methyl-t-butylether before the remainder of the olefin-hydrocarbon stream is directed to a second reaction zone. The second reaction zone is a cracking unit that converts a portion of the olefin-hydrocarbon stream to additional ethylene and propylene using SAPO-34 catalyst.
U.S. Pat. No. 5,914,433 teaches a one catalyst system to increase ethylene yields by cracking the butylene produced during the oxygenate conversion reaction. The cracking of the butylene may take place in a second conversion zone within the oxygenate conversion reactor, or in an external reaction unit. If an external reaction unit is used, the catalyst from the regenerator is used to convert the butylene to additional ethylene. In either case, the same catalyst is used for oxygenate conversion and for the butylene cracking.
GB 2171718 teaches that a zeolite catalyst, particularly a dealuminated, mordenite zeolite can be used to convert an oxygenate to a product containing olefin. The olefin product from this reaction is then separated into ethylene and propylene and a C4+ olefin portion that contains butenes. The C4+ olefin is then recycled back to the oxygenate conversion reactor to produce additional ethylene and propylene. DE 3524890 teaches that ZSM-5 can be used to convert butenes to ethylene and propylene.
Methods are known for increasing the production of ethylene and propylene from a conventional catalyst or steam cracker by a disproportionation or metathesis of the produced butenes. In U.S. Pat. No. 5,026,935, propylene and butenes are directed to a reaction zone containing a metathesis catalyst, e.g., MoOx or WOx, to produce more ethylene. Similarly, in U.S. Pat. No. 5,026,936 ethylene and butenes are directed to a reaction zone using a metathesis catalyst to produce more propylene. U.S. Pat. No. 5,990,369 also describes the potential benefit of providing an olefin metathesis reaction unit to maximize ethylene or propylene yield from an oxygenate feedback.
U.S. Pat. No. 6,048,816 teaches a phosphorous modified ZSM-5 catalyst for the conversion of methanol to light olefin. The catalyst converts methanol to a light olefin stream containing over 30% by weight of ethylene and propylene.
European Patents 0 109 059 B and 0 109 060 B teach the use of ZSM-5, a modified ZSM-5, or ZSM-11 to convert butenes to propylene and smaller amounts of ethylene at temperatures between 400° C. to 600° C.
U.S. Pat. No. 5,043,522 to Leyshon teaches that a feed containing 40 to 95 wt. % paraffin having 4 or more carbon atoms and 5 to 60 wt. % olefins having 4 or more carbon atoms will produce more ethylene and propylene than if the two streams were directed to separate catalytic cracking units. The preferred catalyst for such a zeolitic cracking process are of the ZSM-type, particularly ZSM-5, and borosilicates.
U.S. Pat. No. 4,681,864 discloses that SAPO molecular sieve, particularly SAPO-37 catalyst, has an alternative use as a cracking catalyst. However, Edwards indicates that activated SAPO molecular sieves have poor storage stability.
The references suggest that SAPO molecular sieves are preferred over zeolite molecular sieves for converting oxygenates, particularly methanol, to obtain significant quantities of ethylene and propylene. However, SAPO molecular sieves are reported to have poor storage stability. It would, therefore, be advantageous to obtain significant quantities of ethylene and propylene using catalysts which have good storage stability.